An accurate representation of voiced or mixed type of speech signals is essential for synthesizing very high quality speech at low bit rates (4.8 kbit/s and below). For bit rates of 4.8 kbit/s and below, conventional Code Excited Linear Prediction (CELP) does not provide the appropriate degree of periodicity. The small code-book size and coarse quantization of gain factors at these rates result in large spectral fluctuations between the pitch harmonics. Alternative speech coding algorithms to CELP are the Harmonic type techniques. However, these techniques require a robust pitch algorithm to produce a high quality speech. Therefore, one of the most prevalent features in speech signals is the periodicity of voiced speech known as pitch. The pitch contribution is very significant in terms of the natural quality of speech.
Although many different pitch estimation methods have been developed, pitch estimation still remains one of the most difficult problems in speech processing. That is, conventional pitch estimation algorithms fail to produce a robust performance over variety input conditions. This is because speech signals are not perfectly periodic signals, as assumed. Rather, speech signals are quasi-periodic or non-stationary signals. As a result, each pitch estimation method has some advantages over the others. Although some pitch estimation methods produce good performance for some input conditions, none overcome the pitch estimation problem for a variety input speech conditions.